Which of the following materials is typically tested every 5 years for hydrostatic testing?

The correct choice is aluminum, as this material is commonly used in the manufacturing of medical gas cylinders due to its lightweight and corrosion-resistant qualities. Hydrostatic testing is crucial for ensuring the safety and structural integrity of gas cylinders. This testing involves filling the cylinder with water, pressurizing it, and checking for leaks or any deformation of the cylinder's structure. For aluminum cylinders, the standard recommendation is to perform hydrostatic testing every five years. This interval helps to identify any potentially hazardous conditions before they can lead to failure during use.

In contrast, steel tanks have different testing intervals and procedures. Copper is not typically used for pressure vessels in the same way aluminum or steel is, and composite materials have varying testing requirements depending on their specific composition and intended use. Thus, aluminum's established testing cycle is related to both its properties as a material and the regulations governing the safe use of medical gas tanks.